Thanks for that, the exact same problem, and fix for mine too. The back the back of seat element failed.
The 2 black plugs and sockets that go off to each heating element are the ones with , base of seat brown and red/white cable and the back of seat red/white and red/white cable.
Shorted out those suckers on the back of seat plug and working OK on the base now. Strange how the back of the seat element fails, the base is where your butt is on would get more abuse
Looks a hard job to actually replace the element but having the base working is a 90% fix in my opinion
Note these wiring colours are for UK RHD car and the drivers seat 2004 boxster